1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly, to the improvement of compensation of a radial tilt of an optical disk.
2. Description of the Related Art
In an optical disk apparatus, when an optical disk is inclined in the radial direction with respect to an objective lens of an optical head, the configuration of focused spots on the optical disk fluctuates due to the coma aberration arising from the inclination of the optical disk in the radial direction thereof which is called a radial tilt, so that the read/write characteristics fluctuate. Therefore, it is required to detect and compensate for the radial tilt of the optical disk.
In a first prior art optical disk apparatus (see: JP-A-11-039683 and JP-A-2001-195762), a tilt sensor is provided within an optical head to detect a radial tilt of an optical disk, and an actuator for tilting an objective lens or the optical head in the radial direction of the optical disk in accordance with the output signal of the tilt sensor. As a result, the radial tilt detected by the tilt sensor is brought close to zero.
In the above-described first prior art optical disk apparatus, however, since the tilt sensor is large in scale, it is impossible to decrease the size of the optical head including the tilt sensor. If the tilt sensor is forcibly decreased in size, the sensitivity of the tilt sensor deteriorates.
In a second prior art optical disk apparatus (see: JP-A-9-007207 and JP-A-2000-057606), the amplitude of a tracking error-signal, the amplitude of an radio frequency (RF) signal or a jitter of a digitalized signal of the RF signal is calculated, so that the radial tilt of an optical head is compensated for in accordance with a relationship between the radial tilt of an optical disk and the above-calculated value.
In the above-described second prior art optical disk apparatus, however, since there are a lot of disturbances other than the radial tilt component, a high accuracy of compensation of the radial tilt cannot be expected.
In a third prior art optical disk apparatus (see: JP-A-10-222860 and JP-A-2000-195080), at least one tilt sensor is provided outside the optical head to estimate a radial tilt of an optical disk.
In the above-described third prior art optical disk apparatus, although the problem of the first prior art optical disk apparatus is dissolved, the estimated value of the radial tilt is not accurate, so that a high accuracy of compensation of the radial tilt cannot be expected.
In a fourth prior art optical disk apparatus (see: JP-A-2000-348362), a focus search is carried out to detect periods at inner and outer peripheral sides of an optical disk corresponding to the amount of a radial tilt, and a difference between the periods is calculated to estimate the amount and direction of the radial tilt.
In the above-described fourth prior art optical disk apparatus, however, since the above-mentioned difference is affected by the attitude of the optical disk, and there is a rolling per one revolution of the optical disk, a high accuracy of compensation of the radial tilt cannot be expected.
In a fifth optical disk apparatus (see: H. Yamaguchi et al., “4.7 GB DVD-RAM Drive”, Matsushita Technical Journal Vol. 45, No. 6, pp. 67-73, Dec. 1999), an offset value of a tracking error signal is detected as a radial tilt signal by centering a focused spot at a track using complementary allocated pit address (CAPA) headers.
The above-described fifth prior art optical disk apparatus, however, cannot be applied to optical disks having formats other than the CAPA headers.